The present invention relates to a semiconductor memory device using a capacitor with a ferroelectric thin film.
Recently, memory devices (ferroelectric memories) using ferroelectric thin films as storage media have been developed, and some have already been used in practical applications. The ferroelectric memory is nonvolatile and does not lose its memory contents after the power supply is turned off. When the film is sufficiently thin, spontaneous polarization can be quickly inverted to enable high-speed write and read operations, like the DRAM. Further, a large capacity is also attained because a 1-bit memory cell can be constituted by one transistor and one ferroelectric capacitor.
A ferroelectric thin film suitable for the ferroelectric memory must have large residual polarization, low temperature dependency of residual polarization, a long holding time (retention) of residual polarization, and the like.
At present, as a ferroelectric material, lead zirconate titanate (PZT) is mainly used. PZT is a solid solution of lead zirconate and lead titanium. PZT having a molar ratio of almost 1:1 in the solid solution is considered to be excellent as a storage medium because its spontaneous polarization is large to allow inversion even with a low electric field. Since the transition temperature (Curie temperature) of PZT between the ferroelectric phase and the paraelectric phase is as relatively high as 300.degree. C. or higher, the stored contents are hardly lost by heat within the use temperature range (120.degree. C. or lower) of a normal electronic circuit.
However, a high-quality PZT thin film is difficult to fabricate due to the following reason. First, since lead as the main component of PZT easily evaporates at 500.degree. C. or higher, the composition is difficult to accurately control in sputtering and subsequent annealing. Second, PZT exhibits ferroelectric properties only when it forms a perovskite crystal structure. PZT having this perovskite crystal structure is rarely obtained, and a crystal structure called pyrochlore is easily obtained. When PZT is applied to a silicon device, lead as the main component is difficult to be prevented from diffusing into silicon, and is easily reduced in a reduction atmosphere in the device process to lose the ferroelectric properties.
Barium titanate (BaTiO.sub.3) is known as a typical ferroelectric, other than PZT. Barium titanate also has a perovskite crystal, like PZT, and its Curie temperature is about 120.degree. C. Since barium constituting barium titanate rarely evaporates, compared to lead, the composition is relatively easily controlled in forming a barium titanate thin film. When barium titanate crystallizes, most crystal structures are of the perovskite type.
Regardless of these advantages, a thin film capacitor using barium titanate is hardly examined as the storage medium of the ferroelectric memory because the residual polarization of barium titanate is smaller than that of PZT, and the temperature dependency of its residual polarization is high. This is because the Curie temperature of barium titanate is as low as 120.degree. C. When a ferroelectric memory is formed using barium titanate, its memory contents may be lost upon exposure to a high temperature of 100.degree. C. or higher. In addition, the temperature dependency of residual polarization is high even in the use temperature range (85.degree. C. or lower) of a normal electronic circuit, and the operation is unstable. Therefore, a thin film capacitor using a ferroelectric thin film of barium titanate is considered to be unsuitable as the storage medium of the ferroelectric memory.